Patients with pulmonary arterial hypertension(PAH) suffer from chronic shortness of breath, and have impaired survival related to progressive right ventricular failure. Abnormal vasoreactivity to nitric oxide(NO) plays a role in the pathophysiology of PAH. Phosphodiesterase Type 5 Inhibitor (PDE5 inhibitors) sildenafil have been shown to be beneficial in PAH, but extent of benefit is variable.